Understanding the factors underlying the heterogeneity of social and communicative abilities in children with autism spectrum disorders (ASD) is a critical step toward designing individualized interventions and predicting developmental trajectories. As attention contributes to typical variation in social and communicative development, and attention is often atypical in individuals with ASD, it is possible that attention contributes to the variance in social and communication skills found in young children with ASD, and may predict social- communication growth in early childhood. Indeed, NIH has called for longitudinal investigations of the role of attention in shaping social and communication development in autism. However, whilst joint attention is known to be critical in socio-communicative development in ASD, little is known about the role of other domains of attention in shaping these trajectories. This proposal examines whether performance in a range of attention domains contribute to variability in social and communication skill in young children with ASD, children with neurotypical development (NT), and younger siblings of children with ASD. The study examines the relations between attention and social and communication skill between 9- and 48- months of age. This project involves three studies: (1) Attention and Cardiovascular Reactivity in Children aged 2 to 4 years with ASD and NT. This project will examine the relation between attention and regulation in social- communication skills in early childhood and prediction of social-communication growth through six-month and one-year follow-up assessments. (2) Social-communication and Language Outcomes in Younger Siblings of Children with ASD. In younger siblings of children with autism, the project will examine the relations between attention and regulation in infancy and social-communication and language skills in early childhood. Infant siblings who were initially tested at 6-, 12- and 18-months will be followed up at 36 months. This project will focus on early attention skills and their relation to later attention, regulation, social-communication, and language ability. (3) Attention and Cardiovascular Reactivity from Infancy to Childhood. Future assessments of high-risk populations requires that assessments of critical domains be designed to assess individuals during pre-symptomatic phases, during the emergence of symptoms as well as when stable diagnoses can be formed. This project will examine the stability and reliability of the measures used to characterize the phases of attention, and their contribution to feature attention, spatial attention and joint attention, in both social and non- social domains in a sample of neurotypical infants between 9 and 48 months of age and children with ASD between 24 and 48 months of age. In summary, we propose that measures of attention and cardiovascular reactivity may be predictors of social-communication growth trajectories and indicators of targets for individualized intervention.